1. Field of the Invention
The present invention relates to an electric spindle motor.
2. Description of the Related Art
Motors including fluid dynamic pressure bearing mechanisms have often been used as motors for use in disk drive apparatuses. An example of such a motor is disclosed in JP-A 2001-214929 which includes a shaft, a flange, a sleeve, and a sleeve cap. The shaft is fixed to a base plate. The flange is fixed to the shaft. The sleeve cap is fixed to a bottom end of the sleeve. The shaft is arranged radially inward of the sleeve and the sleeve cap. The flange is arranged between a lower portion of the sleeve and the sleeve cap. Both an upper portion and a lower portion of the shaft include radial dynamic pressure grooves defined therein. Both an upper surface and a lower surface of the flange include thrust dynamic pressure grooves defined therein. A lubricating oil is arranged in a gap defined between a combination of the shaft and the flange and a combination of the sleeve and the sleeve cap. An upper surface of the lubricating oil is arranged between an upper portion of an outer circumferential surface of the shaft and an upper portion of an inner circumferential surface of the sleeve. A lower surface of the lubricating oil is arranged between a lower portion of the outer circumferential surface of the shaft and an inner circumferential surface of the sleeve cap. When the motor is driven, dynamic pressures generated by the radial dynamic pressure grooves and the thrust dynamic pressure grooves act to support the sleeve and the sleeve cap such that the sleeve and the sleeve cap are rotatable about the shaft.
A conventional fluid dynamic bearing apparatus used in a spindle motor disclosed in JP-A 2007-162759 includes a shaft body and a tubular sleeve body inside which the shaft body is inserted. The shaft body is fixed to a base plate of the motor. The sleeve body is fixed to a rotor of the motor. The shaft body includes a first thrust flange and a second thrust flange. The first thrust flange is annular in shape and arranged above the sleeve body. The second thrust flange is annular in shape and arranged below the sleeve body. The first thrust flange is integrally formed with the shaft body and the second thrust flange is fixed to the shaft body through welding or the like. In the fluid dynamic bearing apparatus, a radial bearing portion is defined between the shaft body and the sleeve body. A thrust bearing portion is defined between the sleeve body and each of the two thrust flanges. The sleeve body and the rotor are thereby supported to be rotatable relative to the shaft. In addition, the sleeve body includes a communicating hole arranged to connect two thrust gaps with each other. Surfaces of a lubricating oil are arranged in the vicinity of both an upper end opening and a lower end opening of the communicating hole.
However, the motor disclosed in JP-A 2001-214929 has a problem in that a difference in pressure may occur between the upper surface and the lower surface of the lubricating oil due to errors in processing the dynamic pressure grooves, or errors in assembling parts of the motor. The difference in pressure may lead to leakage of the lubricating oil through one of the surfaces of the lubricating oil. Also, the spindle motor disclosed in JP-A 2007-162759 has a problem in that, when the second thrust flange is fixed to the shaft body, it is difficult to define a minute gap between the second thrust flange and the sleeve body with excellent precision, and that a high-level technique is required when fixing the second thrust flange to the shaft body to ensure sufficient performance of the thrust bearing portion.